Limekilns, mysterious rockpiles, non-meteorites, and how history and geology interact

A limekiln in central Pennsylvania; credit: Lauren Milideo

One afternoon this summer, my husband and I were driving in central Pennsylvania when I spotted something awesome from the

car window (as often happens). I started clapping or pointing to show that we’d just passed something awesome. My husband turned around and drove up the little dirt road I pointed out. What was so awesome? At the time, I didn’t know. Well, I kind of did. The road was called Limekiln Road, so I surmised that I’d spotted a limekiln.

What I was actually looking at was a beautiful, perfectly assembled stone archway in the side of the hill, and under the arch, a little room. Stone walls stood on either side of the arch. The top of the arch was probably about 4 and a half feet off the ground, and the room was maybe 6 feet wide and 5 feet deep. The back and sides of the little chamber were also formed of stones fitted together, and at the back was a small, hollowed-out area. The base of the room didn’t appear to be lined with stone, though small rocks and leaves littered the floor. Lying on the ground surrounding this structure were hunks of what looked like melted glass, and outside the chamber, a little waterfall of cobbles spilled down the remainder of the hill.

A view of the archway; credit: Lauren Milideo

It was old, and it was cool, and it was a mystery.

I’d never seen a limekiln before, so the research started. I knew that iron furnaces were a major industry in this area through the 18th and 19th centuries, and that lime was a needed “ingredient” for that industry. I found great descriptions of limekilnshere, here, and here, which basically said that, in order to obtain the materials necessary for industrial or agricultural use, you need to start off with limestone, and then burn it. (In a kiln, for example.) Depending on what you had available to you, either wood or coal could be used as fuel. The limestone would be dumped into the top of the kiln (which would be about 10-20 feet high), and a fire could be started either off to the side of the kiln’s central shaft (for indirect heat, which would keep ash out of the lime) or directly in the kiln, on a grate. The finished lime would begin to fall to the bottom of the kiln after about 36 hours (though one source said a fire could burn for weeks) at which point the lime could be removed. In some cases, it was in the form of large pieces of rock; in others, it was more a powder. From here, the lime was removed to another area for cooling (and possibly crushing) and transported elsewhere for industrial or agricultural use.

So people are hauling rocks to build the kiln. Then they’re hauling other rocks to burn in the kiln. They’re hauling wood or coal to burn it with. Why? What is lime used for? There’s an extensive and helpful explanationavailable from the University of Wisconsin. But for our purposes (since we’re dealing with the historic iron furnace industry), the important thing that lime does is bond with silicates. If you’re working with iron ore, that’s important because you need to separate the iron from the other materials in the ore – this is how you obtain iron that you can use for industrial and other purposes. To learn more about the overall function of a historical iron furnace (including its limekiln), check this out.

So with a place like this, you can figure out a lot more than you might think. If you see a lime kiln, you instantly know something about the environment as it once was, and about the geology beneath your feet. Lime kiln? There’s limestone nearby. That means that at some point, this place was likely a marine environment. And you’d need fuel for your kiln. So maybe you were shipping in coal, but there’s a good chance there was a lot of wood to burn. So even if there’s a subdivision now, at some point it was likely a forest. The relationship between history is often deep, rich, and fascinating.

Here are some questions that I had, and answered, and in the course of my research:

1. Why is the kiln in the side of a hill? (Because the shaft was – I’m guessing – about 15 feet high, and when the stone “chimney” was completed, the whole thing was covered in soil to insulate it and keep it hot while the lime was being burned.)

2. What was I actually looking at? (I think it was the front of the kiln, where they’d be removing the hot, finished lime.)

3. What was that little cubbyhole in the back of the chamber? (I think it was the area where the finished lime would fall, into which shovels or sticks would be inserted to pull it out.)

4. What was the glassy stuff on the ground? (It was slag, which is a leftover from the lime-making process. It was also not a meteorite.)

5. Did I see the whole thing? (Based on what I’ve read, not even close. I think I just saw the front. I didn’t see the top, where the hole was, through which the limestone would be dumped. I didn’t see any holes in the side for putting in the fuel to burn. Perhaps any remaining holes have been filled in or covered over. I wanted to go up onto the top and see what was there, but the hill was covered in very dense vegetation, it was summer, I was wearing shorts, and I didn’t want to emerge covered in Lyme-carrying ticks. I know. I’m a wuss.)

I think the little cubby in the back is where the lime emerged for removal following burning. credit: Lauren Milideo